Cyclones are commonly used devices for separating particles from gas streams, such as dust in stack emissions and the like. Cyclones typically admit the gas stream in such a manner that a vortex is created. As a result of centrifugal force, the entrained particles in the swirling gas stream are flung against the inner walls of the cyclone and are segregated out. The cleansed gas stream leaves the cyclone through the vortex finder while the particles are funneled into a hopper or the like.
Unfortunately, the collection efficiency of conventional cyclones is relatively low. This is especially true when there is a high percentage of fine particles in the gas stream because the resulting centrifugal forces are smaller and often insufficient to segregate the particles. Even heavier particles encounter flow turbulence and secondary flows which tend to bounce them off the cyclone walls, thereby causing re-entrainment.
Many variations in cyclone designs have been suggested to improve cyclone efficiency. These include the use of acoustic agglomeration to increase particle size, water injection to reduce particle bounce off the walls, electrostatics to increase the driving force of particles to the walls, and once-through cyclone designs to eliminate secondary re-entrainment. None of these previous refinements has achieved the performance needed to make cyclone collection efficiency comparable to baghouses, electrostatic precipitators, or other counterpart devices. On the other hand, the other devices are much larger and carry higher capital costs.
There have been efforts to enhance the mechanical separation of a cyclone with electrostatics. For instance, U.S. Pat. Nos. 1,381,719 issued to McGee et al. and 2,360,595 issued to Thompson both disclose an electrostatically enhanced cyclone design. Several problems persist, however. Particles in these devices are initially distributed all over the device cross-sectional area between the discharge and collecting electrodes. In this case, fine particles situated closer to the separator core may not have enough time to reach the walls and, therefore, leave the separator together with the clean gas. Also, when the particles are quite small and their concentration in the feed stream is high, the device may experience the corona suppression problem.
Secondly, the existing electrostatically enhanced cyclones are employed as single-stage units, where ionizing and separating processes are accomplished in the same vessel. This does not allow the highest separation efficiencies and minimum power consumption to be achieved.
Third, the existing electrostatically enhanced cyclones can work as particulate collectors or separators. In the first case, secondary flows (strong toroidal vortices) significantly impair the collector performance. In the second case, the particles are extracted from the device together with some bleed flow, and complete gas/particle separation can be accomplished only if the bleed flow is treated additionally in the downstream collector. If the downstream collector efficiency is low, the efficiency of the system, which incorporates both the separator and downstream collector, is also low.
Fourth, particles on the collecting electrodes of the existing electrostatically enhanced cyclones may form an immovable particulate layer, and as a result the devices may experience back corona problems and re-entrainment of the particles in the clean stream.
It would be greatly advantageous to design an electrostatically enhanced separator without shortcomings of the existing state-of-the-art devices. This separator would be very compact and able to achieve the highest collection efficiency.